This invention concerns a process for determining the carbon black concentration and distribution in rubber compounds and other carbon black-containing materials using pulsed laser beams focused on the material surface each of which produce a plasma with a radiation characteristic of the elements or molecules contained therein and divide the surface with their end regions into grid areas in which are located measuring points formed by the laser beam focuses, whereby the characteristic radiation, spectrally dispersed in the form of spectral lines or molecule bands, is measured by a detector unit and whereby from the concentration values calculated by reference to numerical ratios from the radiation intensities of selected elements/molecules with subsequent storage and allocated to the relevant measuring points, the concentration value curve at least over a section of the surface is established.
In one aspect, the present invention lies in having perceived, as an essential part of a tire manufacturing process, the technical problem of measuring the carbon (C) associated with the carbon black separate from the carbon of the rubber material, particularly and preferably based on different responses to ignition.
According to this aspect, the invention also concerns a process for manufacturing a pneumatic tire for vehicle wheels, substantially comprising the steps of preparing rubber compounds for a plurality of semi-finished components, of preparing semi-finished components from said compounds and of assembling together said components in a raw carcass, according to which the elemental carbon from carbon black contained in at least one of said compounds, or semi-finished components, is quantitatively detected on-line, thus monitoring in real time the dispersion degree and the homogeneity of the carbon black in said at least one rubber compound, so as to control and, if needed, to vary, on the basis of said detected information, the quantity of carbon black which is going to be mixed with polymeric materials and other ingredients for preparing said at least one rubber compound.
The invention relates further to a suitable device to perform the process for determining the carbon black concentration and distribution in-rubber compounds and other carbon black-containing materials, comprising a facility for producing briefly maintained laser beams, a first optical assembly lying in their axis consisting of a first lens arrangement and a first deflector unit for aligning the laser beam end regions--whose laser beam focuses in each case form a measuring point--relative to the material surface, a second optical assembly with a second lens arrangement exhibiting a second deflector unit, by which second lens arrangement the radiation from the plasma produced in the laser beam focus can be supplied to a spectrograph with detector unit to identify the radiation constituents, and a computer serving as evaluator connected with the detector unit, whereby the plasma is evaluated before the next laser pulse is actuated and from the concentration values stored in the computer--which have been calculated by comparison with corresponding ratios from specimens of known composition by reference to numerical ratios from the measured radiation intensities of selected elements/molecules and are assigned to various measuring points--the concentration value curve at least over a section of the surface is established.
A process and a device of the above mentioned kind are known from DE-A1-40 04 627; there, the surface to be examined is divided by a relative movement between itself and the end region of the laser beam into grid areas which are each assigned to a measuring point formed by the laser beam focus. To be able to gain information on the degree of mixing and the degree of concentration and dispersion of the constituents contained in the material, the number of measuring points in one section of the surface can be adjusted in several gradations by altering in a suitable way the pulse frequency of the laser unit and/or the movement of the measuring point. Depending on whether it is intended to examine the degree of overall material homogeneity or the degree of dispersion on the surface, the spacing between the measuring points assigned to the grid areas is relatively large or relatively small; this difference is due to the fact that the aim of investigating the degree of dispersion is to gain information on fine, small-area constituents such as for example salt crystals.
Underlying the invention is the task of configuring the known process and the known device--which are based on the TRELIBS principle (Time-Resolved Laser-Induced Breakdown Spectroscopy)--in such a way that the concentration and distribution of carbon black in rubber compounds and other carbon black-containing materials, particularly in tire compounds, can also be determined.
It is known that in making vehicle tires a raw carcass is first provided, which carcass is achieved by successively assembling together several different semi-finished components consisting of rubberized fabrics, reinforced with textile or metallic cords, and total rubber components.
By way of example, when the carcass for a tubeless tire is to be produced, that is a carcass for tires that in use do not require the presence of an inner tube, the main components include a so-called "liner" that is a layer of elastomeric air-proof material, a carcass ply, a pair of annular metal elements, commonly referred to as bead cores, around which the opposite ends of the carcass ply are folded, as well as a pair of side pieces made of elastomeric material, which will constitute the sidewalls of the tire, extending over the carcass ply at laterally opposite positions.
The thus assembled carcass is then submitted at least to the application of a so-called "belt pack" and of a tread band, provided for being imprinted with an appropriate tread pattern: the completed raw tire is then ready for undergoing the final vulcanization process for the purpose of obtaining a finished tire.
The process to make the raw carcass, substantially starts with the step, made in a so-called hambury, of preparing a rubber compound by mixing together at least a polymeric material with other ingredients, including carbon black, according to prefixed quantitative ratios between said ingredients established in a given recipe; subsequently, from said compound at least a semi-finished component for said tire is formed, like the tread band or the sidewalls or other rubber components.
In a process "of continuous type" the forming devices, like the extruders or calendars and so on, quite conveniently, are directly feeding the tire building apparatus, on which a plurality of said semi-finished components are assembled together to build up a raw carcass, subsequently molded and vulcanized in a curing mold.
The advantage of this kind of process is that to minimize the need to store and manage a lot of semi-finished components, thus avoiding storage costs and possible problems in respect of feeding the requested components to the tire building apparatus. It is then easy to appreciate the importance to check and measure, as soon as possible, and in real time during said building process, the composition of the corresponding compounds, i.e. the concentration and the degree of dispersion of the various ingredients, including the carbon black, in said compounds.
A check on the semi-finished components coming from a store, just before their assembling in the raw carcass, or during their manufacture according to said continuous process, notwithstanding is possible, in case a wrong composition of the related compounds should be detected, would require to restart with the preparation of said rubber compounds and the corresponding components, with waste of time and money, and possible great disadvantages on the flow of the semi-finished components during the tire building process.
Accordingly, the preferable time and place at which to have said check done is just when and where the compound is coming out of the bambury: by this way it is possible to immediately vary, on the basis of said detected information, the quantity of carbon black and other ingredients which are going to be loaded in the bambury to be mixed with said polymeric materials so as to fit the established composition for said compounds according to the prefixed quantitative ratios of ingredients of the given recipe, thus avoiding the preparation of possible batches of rubber compounds and/or a lot of semi-finished components not respondent to the prescribed provision.
In the following we will call as "on-line process" a manufacturing process in which the control on the product is made during the process. In the above, it has been proved to be of the greatest importance to check the concentration and distribution of the carbon black content in said tire rubber compounds.
The carbon black content influences above all the material hardness of such rubber compounds and therefore provides information on certain properties and the quality of the rubber compound to be examined. Determining the carbon black content has hitherto proved difficult insofar as this constituent can only be measured indirectly and only through laboratory tests, not allowed in an on-line process. Thus, it is an object of this invention to provide a device and process for measuring directly carbon black in an on-line process.